1. Field of the Invention
The present invention relates to a control device for trimming knives in flat-bed sewing machines of the type comprising a control eccentric mounted on a rotating shaft and operatively engaged with a connecting rod acting upon a rocking lever oscillatably engaged on a fixed pivot and provided with an elongate shaped opening having a curvilinear portion extending concentrically with the axis of the fixed pivot and an adjoining engagement seat portion extending substantially radially to the fixed pivot; a drive group for a movable knife operated by the rocking lever through a connecting member linked to the latter by means of a movable pivot slidably passing through the elongate shaped opening; and a disengagement group comprised of a disengagement lever supported by a control spindle and provided with an elongate guide opening through which a side portion of said movable pivot is slidably engaged, said disengagement lever being oscillatable, through the rotation of said control spindle, from an engagement position in which the movable pivot is in engagement with the engagement seat portion of said elongate shaped opening and the elongate guide opening extends concentrically with the axis of the fixed pivot, to a disengagement position in which the movable pivot is in engagement with the curvilinear portion of the elongate shaped opening.
2. Prior Art
It is known that industrial flat-bed sewing machines are often provided, for the accomplishment of particular workings, with trimming knives acting through the work-supporting table to carry out a cutting action on at least one of the fabrics being worked during the execution of a seam.
In greater detail, the cutting or trimming of a fabric is carried out by virtue of the combined action of a fixed knive disposed substantially flush with the work-supporting table through an opening formed in the latter and a movable knife oscillating through the opening itself in a substantially vertical direction.
The movable knife is driven by a control device enclosed in the sewing machine bed under the work-supporting table. Such a device summarily comprises a drive group imparting the oscillatory motion to the movable knife through an eccentric mounted on a rotating shaft and with which a connecting rod acting on a rocking lever, is associated. A connecting member is engaged on either side to the rocking lever and to an arm carried by a countershaft to which the movable knife is fastened. In this way the oscillatory motion imparted to the rocking lever by the connecting rod is transmitted to the movable knife through the above mentioned connecting member, arm and countershaft.
The control device further comprises a disengagement group enabling an operator to prevent the knives from working during the operation of the sewing machine.
The above mentioned connecting member is connected to the rocking lever by means of a movable pivot in engagement through an elongate shaped opening formed in the lever. Said elongate shaped opening has a curvilinear portion extending concentrically with the pivoting axis of the rocking lever and an adjoining engagement seat portion disposed substantially radially to the above mentioned axis.
One side portion of the movable pivot is also engaged in an elongate guide opening formed in a disengagement lever supported by a control spindle and said movable pivot is fixed in an axial direction by means of an enlarged end abutting against the connecting member and a screw threaded element abutting against the disengagement lever.
The spindle supporting the disengagement lever has an arm connected by a Bowden cable to a knee control mounted externally to the sewing machine frame. Acting upon the knee control it is possible to rotate the spindle angularly; as a result, the disengagement lever moves the movable pivot along the elongate shaped opening from an engagement position in which the pivot is engaged with the engagement seat portion to a disengagement position in which it is engaged with the curvilinear portion of the elongate shaped opening.
When the movable pivot is in its engagement position it is involved in the oscillations of the rocking lever causing the operation of the movable knife via the members forming the rest of the drive group.
On the contrary, when the pivot is engaged with the curvilinear portion of the elongate opening it is not subjected to perform any oscillation and the movable knife remains therefore inactive.
Although the devices of the above described type can perfectly perform the functions they are intended for, they involve some problems when, due to particular workings to be carried out, they have to be removed from the sewing machine, being then necessary to put them in place again after some time.
These problems originate from the fact that such devices, after being mounted on a sewing machine, cannot be removed therefrom if the above mentioned control eccentric is not first removed. As a result when the device has to be mounted again, it takes a long time to carry out the eccentric timing in order to make the trimming knives operate in synchronism with all the other members of the sewing machine.